Petroleum feedstocks are characterized by relatively high levels of contaminants, including sulfur, nitrogen, Conradson carbon residue, aromatic compounds and metals such as nickel, vanadium and iron. During catalytic hydroprocessing heterogeneous catalysts are contacted with a feedstock in the presence of hydrogen under conditions of elevated temperature and pressure to reduce the concentration of the contaminants in feedstocks. The hydrotreating process promotes reactions such as hydrodesulfurization (HDS), hydrodenitrogenation (HDN), Conradson carbon removal, hydrodemetallation (HDM) and aromatics saturation, accompanied by a boiling shift to lower boiling products. As the sulfur and nitrogen components are converted into hydrogen sulfide and ammonia, metals are deposited onto the catalyst. The results include producing ecologically clean hydrocarbon products such as fuels and protecting other downstream refining catalysts from deactivation.
Processes for removing heteroatoms from feedstocks are known in the art as are catalysts for such removal. Typically, hydroprocessing catalysts contain Group VI and/or Group VIII active metal components supported on a porous refractory oxide such as alumina, alumina-silica, silica, zeolites, titania, zirconia, boria, magnesia and their combinations. Such catalysts are often prepared by combining the active metals with the support. The supports, containing metal components, are typically dried and calcined at the temperatures ranging from about 370.degree. C. to 600.degree. C. to eliminate any solvent and to convert metals to the oxide form. The calcined metal oxide catalysts are then typically activated by contacting with a sulfur containing compound such as hydrogen sulfide, organic sulfur compounds or elemental sulfur to convert metal oxides into catalytically active metal sulfides.
An important and continuing aim in the refining catalyst art is to develop new high performance hydroprocessing catalysts in order to obtain high quality oil products and improve refinery economics. Variations in compositional characteristics or methods of preparation of hydroprocessing catalysts have been attempted to reach these objectives.
It is known in the art that uncalcined catalysts usually provide higher dispersion of active components thereby improving hydrotreating activities. It is essential for the uncalcined catalysts that the active components, such as Group VI and/or Group VIII metal compounds, and promoters such as phosphorous, are not converted into oxide form during a high temperature step. That is, the active compounds are maintained without chemical decomposition until sulfurizing. For example, U.S. Pat. Nos. 5,198,100, 5,336,654 and 5,338,717 disclose a method for preparing a hydrotreating catalyst by impregnating a refractory support with a salt of Group VI metals and with the Group VIII metal heteropolyacids. The catalyst is not calcined or subjected to high temperatures, thereby retaining the heteropolyacids in the original form on the support; however, complete moisture removal from the catalyst during a high vacuum drying step is required before the catalyst is sulfurized.
In general, as the activity of an uncalcined catalyst is increased, the hydrotreating conditions required to produce a given oil product become more mild. Milder conditions require less capital investment to achieve the desired product specifications, such as allowed levels of sulfur, nitrogen, Conradson carbon residue, metals and aromatics, and the catalyst's life is extended due to lower coke formation and other factors.
It has surprisingly been found that preparation of an uncalcined catalyst using a combined volatile content reduction-sulfurizing step allows for the catalyst to be prepared at lower temperatures, in less steps and without calcination, resulting in a catalyst with excellent hydrotreating activity and stability.